1. Field of the Invention
The present invention relates to a computer readable storage medium storing a program, an image processing apparatus and an image processing method.
2. Description of Related Art
In recent years, due to heightening of security consciousness, approaches for preventing images of paper documents from being copied unauthorizingly are carried out and various techniques and methods of such approaches are being developed. Here, “copy” means duplication in which an image of a paper document is read by an optical processing to form a similar image on other paper after digitalizing the read image. This definition applies in the description hereinafter.
As a method for preventing images of paper documents from being copied unauthorizingly, there is a method of creating a paper document in which a tint block image is embedded by providing a tint block print function in an image forming apparatus which forms the images on papers.
Here, the tint block image is an image which is expressed by a collection of dots by using dots having at least two types of sizes. The tint block image is embedded in a document image so as not to be recognized by naked eyes or so as to be difficult to recognize visually. However, when the document image is copied, a portion of the dots are lost or emphasized after copying of the image due to the relation between resolution level of the image reading device and the size of the dots. In such way, the tint block image is manifested as a pattern which can be visually recognized clearly. For example, an image in which a letter string such as “duplication”, “copy” or the like is formed is suggested as such pattern image. When a user sees a paper in which an image including the above pattern image is formed, he/she can easily determine that the image is formed by copying.
Generally, the tint block print function is realized by using a dedicated hardware (for example, a dedicated chip), and the hardware for the tint block print function is pre-provided in an image forming apparatus as shown in JP2006-119735, for example. The image forming apparatus in which the hardware dedicated for the tint block print function is provided is expensive comparing to an image forming apparatus without the tint block print function. Therefore, the tint block print function has been a function to be loaded to image forming apparatuses and multi functional printers of high-end which are relatively expensive.
Meanwhile, in recent years, image forming apparatuses and multi functional printers which are not provided with the tint block print function by the dedicated hardware (hereinafter, described as “low-cost machine”) which are inexpensive comparing to the above image forming apparatuses of high-end are prevailing. At the same time, there are market needs for a low-cost machine provided with the tint block print function.
However, when the tint block print function by a dedicated hardware is to be provided to the low-cost machine, there are problems that the cost will be expensive and that the cost advantage of the low-cost machine reduces or disappears.